In a wind generator, the main shaft couples the rotor hub with a gear, which drives a generator for the production of electricity. The main shaft is frequently supported by friction bearings or sliding bearings. There are mainly two types of friction bearings, hydrostatic bearings and hydrodynamic bearings. Hydrostatically lubricated bearings apply an external oil pump to establish a sufficient lubricating film in the lubricating gap. In a hydrodynamic friction bearing, the pressurized oil film between the bearing surface and the journal is maintained by a hydrodynamic effect, which is due to the rotation of the journal or shaft. However, upon start-up or shut-down of the rotation of the shaft in a hydrodynamic friction bearing, the lubrication between the sliding surfaces can be insufficient. There is a certain critical speed of rotation, which has to be reached in order to establish a lubricating film having a minimum thickness. Upon start-up or shut-down, for example of a wind generator comprising a hydrodynamic friction bearing for supporting the main shaft, this can lead to mixed friction states. These, however, cause wear in the basically wear-free friction bearing. A contact of the sliding surfaces cannot be avoided in all situations. In addition to this, at low speeds of rotation of the shaft, there is no play-free support of a main shaft, because the hydrodynamic friction bearing is generally not play-free at low rotational speeds.
In conventional shaft bearings, in particular for support of the main shaft in a wind generator, the hydrodynamic friction bearings are operated as hydrostatic friction bearings during the start-up and shut-down period. This is to avoid a contact between the sliding surfaces and to minimize the play in the bearing. However, this requires a high-pressure lubricating system for supplying lubricant to the lubricating gap of the bearing. This represents a technically complex and sometimes cost-intensive solution.
Document U.S. Pat. No. 3,708,215 discloses a hybrid bearing comprising a friction bearing and a rolling bearing. The bearing load is shifted from the rolling bearing to the friction bearing and back to the rolling bearing when the supported shaft starts and stops rotating. The transition between the rolling bearing and the friction bearing is performed by raising or lowering a pressure in the lubricating gap of the friction bearing. An active oil pressurization system is, however, needed, which also represents a technically complex and rather costly solution.
It is therefore desirable to have a less complex, reliable and economic shaft bearing. In particular the bearing should be suitable to take up high loads, for example when it is applied to support a main shaft in a wind generator.